This invention relates generally to hydrocarbon solutions of methylaluminoxane compounds and more specifically to such solutions which contain a straight chain trialkylaluminum compound which improves the solubility of the methylaluminoxane so as to permit the preparation of stable, colorless solutions.
Hydrocarbylaluminoxanes complexed with transition metal compounds have been found to be very effective olefin polymerization catalysts (Manzik et al. U.S. Pat. No. 3,242,099). Methylaluminoxane is an especially effective catalyst component. However, it has poor solubility in aliphatic hydrocarbon solvents which are preferred catalyst solvents because they are less toxic than aromatic hydrocarbons. Manufacturers of polymers which may come in contact with foodstuffs are concerned about solvent residues in their products and therefore seek to avoid the use of aromatic solvents during polymer production. Even in aromatic solvents, methylaluminoxane is not completely soluble such that the solutions become cloudy upon standing. Japanese application 63-87717 discloses the use of branched chain alkylaluminum compounds to improve solubility. Branched chain alkyl groups are known to have enhanced solubility in hydrocarbon solvents compared to straight-chain alkyl groups.
Surprisingly, we have found that clear solutions of methylaluminoxane (MAO) compositions in hydrocarbon solvents and especially in aliphatic hydrocarbon solvents can be prepared by dissolving methylaluminoxane and a straight chain trialkylaluminum compound which contains as few as two carbons per alkyl group, i.e. the readily available triethylaluminum (TEA), as well as higher alkyl groups such as tri-n-octylaluminum, in the solvent. Use of this latter compound permits the preparation of aromatic hydrocarbon free, clear solutions while adding only a fraction of aluminum values to the mixture (aluminum alkyl to aluminoxane aluminum mole ratios of about 1:10 or less). This provides a more economical solution to the methylaluminoxane solubility problem and minimizes the amount of aluminum residual in the polymers derived from these catalyst systems.
Further, we have found that adding straight chain trialkylaluminum compounds can also improve the clarity of aromatic solutions of methylaluminoxanes. Such solutions although initially clear, tend to form gels or particulates upon standing.